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Abstract

Influenza A viruses are capable of causing disease in several species, including birds, humans and swine. Host specificity of the viruses is not absolute, and is influenced by a range of factors. Swine play a pivotal role in the interspecies transmission of influenza A viruses, as they are susceptible to infection with both human and avian strains and have been implicated as a “mixing vessel” for the reassortment of influenza A viruses from different species. The reassortment of influenza A viruses of human and avian origin led to human influenza pandemics in 1957 and 1968.

The dynamics of swine influenza viruses in North America changed drastically with the introduction of the avian-origin PA and PB2 and human-origin HA, NA, and PB1 gene segments and the creation of the triple reassortant swine virus lineage in 1998. While the previously circulating classical swine H1N1 influenza virus lineage was very stable in the swine population, triple reassortant lineage viruses have supplanted the classical H1N1 lineage and undergone repeated reassortment events, acquiring HA and NA genes from human, swine, and avian influenza viruses, while maintaining triple reassortant internal gene (TRIG) cassette. Viruses of the triple reassortant lineage have been very successful in the swine population, yet the mechanisms underlying their unique characteristics and increased fitness have not been elucidated.

Here we address the pandemic potential of triple reassortant swine influenza A viruses, their transmissibility, and their relative fitness compared to classical and double reassortant swine influenza viruses. Several triple reassortant viruses, including one with avian-origin HA and NA, were characterized in the ferret, which is a commonly used model for human influenza infection. The effect of the TRIG cassette on the reassortment potential and temperature sensitivity of swine influenza viruses was determined in cell culture, and the replication and transmission of a classical and a reassortant swine virus were compared in pigs.

We found that triple reassortant swine viruses replicated efficiently in the ferret model, although there was some variation in transmission efficiencies. An H2N3 virus with avian-origin HA and NA was transmissible in the ferret model, and this transmissibility could be abolished with a single amino acid change in the HA protein that altered its receptor binding specificity. Avian H2N3 viruses were also capable of replicating in ferrets without adaptation and could acquire transmissibility through a change in the receptor binding specificity of the HA protein.

Both double and triple reassortant swine viruses had an advantage over the classical H1N1 swine virus at early timepoints in cell culture. Reassortant viruses also demonstrated less temperature sensitivity than the classical H1N1 swine virus. The triple reassortant H1N1 virus had an increased reassortment potential in cell culture compared to the classical swine H1N1 virus as determined by acquisition of a human HA gene.

Triple reassortant swine viruses have an increased ability to establish infection, and an increased potential for reassortment, potentially introducing novel HA genes into a host population. This indicates that triple reassortant swine viruses may have an increased potential to cause human pandemics. In April 2009, a novel H1N1 pandemic virus containing five of the six genes of the TRIG cassette emerged in the human population, emphasizing the importance of reassortant swine influenza A viruses in the generation of human pandemics.